In a drop-in type of exhaust gas recirculation (EGR: Exhaust Gas Recirculation) valve that is fitted to an attachment unit to be communicated with an exhaust gas circulation passage of an internal combustion engine (engine) and that has an exhaust gas passage to be communicated with the exhaust gas circulation passage to control the amount of exhaust gas flowing through the exhaust gas passage, a high-temperature exhaust gas flows in through the exhaust gas passage when part of the exhaust gas exhausted from the engine is recirculated. However, a guaranteed heat resistant temperature is provided for main parts of the EGR valve, and thus, when the high-temperature exhaust gas flows therein, it is required to meet the guaranteed heat resistant temperature.
Therefore, in order to protect the main parts of the EGR valve from heat, as shown in FIG. 7, a water-cooled circuit 102 is formed around a housing 101 in a conventional EGR valve. As shown in FIGS. 7(a) and 7(b), a nipple 103 for connecting a cooling water hose (not depicted) that renders the flow in/out of cooling water into the water-cooled circuit 102 is press-fitted into the water-cooled circuit 102 formed in the EGR valve. As shown FIG. 7(c), the water-cooled circuit 102 is formed linearly around the housing 101.
In order to protect the main parts of the EGR valve from heat, it is contemplated to dispose the water-cooled circuit in the vicinity of the main parts. However, in the drop-in type of EGR valve, a drop-in portion of the EGR valve is fitted in an attaching hole of an attachment unit, and thus, it is difficult to dispose the water-cooled circuit in the drop-in portion; therefore, when the main parts are disposed at the drop-in portion, it could not be achieved to produce a sufficient cooling effect on the high-temperature exhaust gas flowing through the exhaust gas passage.
Therefore, there is known an EGR valve in which a water-cooled circuit is formed in an attachment unit to which a drop-in portion of the EGR valve is fitted, as measures for cooling the drop-in portion (for example, see Patent Document 1). In Patent Document 1, it is configured that the water-cooled circuit is established around an attaching hole formed in the attachment unit, and that cooling water is flowed into the water-cooled circuit to thereby produce a cooling effect on even the drop-in portion of the EGR valve fitted to the attachment unit.